1. Technical Field
This invention relates to the technology of making light-weight metal cellular structures and particularly to the use of cold-gas spraying techniques for achieving such metallic cellular structures.
2. Discussion of the Prior Art
Foamed metals have been heretofore made by essentially adding a gas-evolving compound to molten metal and thereafter heating the mixture to decompose the compound causing the gas evolved to expand and foam the molten metal (see U.S. Pat. Nos. 3,940,262; 5,281,251; 5,622,542). To avoid adding the gas-evolving compound to a molten body of metal, solid particles of the metal, mixed with a gas-evolving compound, can be hot pressed or compacted and then subsequently heated near the melting temperature of the metal, or into the solidus-liquidus range of the metal, to create foaming for a cellular structure (see U.S. Pat. No. 5,151,246). Casting molten metal around granules which are then leached out leaving a porous structure is another method of providing metals with cellular structures.
None of the above techniques are compatible with providing a preformed substrate with a foamed metal coating. Among the techniques used to obtain thick coatings on metal members, are thermal-spray depositions, such as plasma jet or electric-arc metal spraying which presents several drawbacks: unfavorable high thermal and dynamic effects on the substrate; unfavorable changing of the physical properties of the coating during spraying; unfavorable phase transformation of the deposited particles; overheating of the substrate; and erosion or jamming of the spraying equipment.
What is needed is a relatively low temperature spraying technique that achieves compacting of the sprayed metal particles and yet has a composition that promotes foaming to achieve a cellular deposit. xe2x80x9cCold-gasxe2x80x9d dynamic spraying of metals was initiated in Russia relatively recently as evidenced by their U.S. Pat. No. 5,302,414. However, the disclosure of such Patent did not teach how to achieve a foamed structure nor did it teach suitable parameters to obtain metal particle welding commensurate with the need for metal foaming.
The invention herein creates a cellular, venticular or foamed metal structure by first depositing a coating of cold-compacted metal particles, containing a foaming agent, onto a suitable substrate as a result of at least sonic-velocity projection of such particles, and, secondly, thermally treating the coating to gasify the foaming agent and thermally transform the welded metal particles to a plastic-like condition, such as a result of the temperature being slightly above the softening temperature for the metal or above the eutectic temperature of the metal if it is such an alloy.
In a more definitive aspect, the invention is a method of fabricating a foamed metal structure using a supply of metal particles, comprising: (a) introducing a supply of powder metal particles and foaming agent particles into a propellant gas to form a gas/particle mixture; (b) projecting the mixture at or above a critical velocity of at least sonic velocity onto a metallic substrate to create a deposit of pressure-compacted metal particles containing the admixed foaming agent; and (c) subjecting at least the coating on said substrate to a thermal excursion effective to activate expansion of the foaming agent while softening the metal particles for plastic deformation under the influence of the expanding gases.